一个Java例子，解释清楚注解的作用

https://baijiahao.baidu.com/s?id=1612408653409570352&wfr=spider&for=pc

Table注解

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.TYPE) // 可以应用于类的任何元素
@Retention(RetentionPolicy.RUNTIME) //:在运行时有效（即运行时保留）
public @interface Table {
    String value();
}

Column注解

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.FIELD) // 可以应用于类的任何元素
@Retention(RetentionPolicy.RUNTIME) //:在运行时有效（即运行时保留）
public @interface Column {
    String value();
}

Person类

package com.raykeyzzz.annotation;

@Table("Person")
public class Person {
    @Column("name")
    private String name;
    @Column("userName")
    private String userName;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }
}

Main类

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class Main {
    public static void main(String[] args) throws InvocationTargetException, IllegalAccessException {
        Person p1 = new Person();
        p1.setName("ai");
        p1.setUserName("aiai");
        String query = query(p1);
        System.out.println(query);

    }
    public static String query(Person person) throws InvocationTargetException, IllegalAccessException {

        StringBuilder sb = new StringBuilder();
        // 通过反射获取对象
        Class p = person.getClass();
        // 判断class类是不是注解类
        boolean exist = p.isAnnotationPresent(Table.class);
        if (!exist) {
            return null;
        }

        // 如果是强制转化为Table
        Table table = (Table) p.getAnnotation(Table.class);
        String tableName = table.value();

        // 接下来就是拼接sql语句
        sb.append("select * from ").append(tableName).append(" where 1=1");
        Field[] fArray = p.getDeclaredFields();
        for (Field field : fArray) {
            boolean fExist = field.isAnnotationPresent(Column.class);
            if (!fExist) {
                return null;
            }
            Column column = field.getAnnotation(Column.class);
            String columnName = column.value();
            String fieldName = field.getName();
            Object fieldValue = null;
            //此处将生成getXXX方法，用于下面通过反射执行对应方法拿到里面的返回值
            String getMethodName = "get" + fieldName.substring(0,1).toUpperCase() + fieldName.substring(1);
            try {
                Method method = p.getMethod(getMethodName);
                fieldValue = method.invoke(person);
            } catch (NoSuchMethodException e) {
                e.printStackTrace();
            }

            sb.append(" and ").append(columnName).append("=").append(fieldValue);
        }

        return sb.toString();
    }
}


``